1. Field of the Invention
The present invention relates generally to a device for the photoelectric determination of the position of at least one focal plane of an image with an optical correlator.
2. Background of the Invention
Photoelectric position sensing devices are known in the art. U.S. Pat. No. 4,071,297 describes a process and apparatus for the measurement of distances by means of an optical correlator comprising an optical imaging means, a grating structure arranged in the vicinity of the image plane of the optical means to act as a spatial frequency filter and at least one photoelectric receiver system coordinated with it. In order to measure the distance of an object, i.e., to determine the position of the image focal plane, the grating structure jointly modulates a plurality of luminous fluxes, the fluxes passing through different areas of the entrance pupil of the optical imaging means. The fluxes are split, either by geometric or physical means or through an additional modulation corresponding to the respective areas of the pupil. The fluxes are then applied, either successively to a common photoelectric receiver, or simultaneously to separate photoelectric receivers. The output signals of the receivers serve to actuate a display and/or readjustment device.
The primary disadvantage of the device described in U.S. Pat. No. 4,071,297 is to be found in the fact that variations, for example, mechanical or thermal variations, within the apparatus may lead to signal deterioration or even signal falsifications. For this reason, it has already been proposed in U.S. Pat. No. 4,037,958 to provide an optical sensor in an apparatus as described hereinabove with regard to U.S. Pat. No. 4,071,297, capable of responding to such variations, which result, e.g., from thermal effects, for producing electrical signals which can be used to indicate and/or to correct the variations and hence their optical effects.
To determine the position of the focal plane of an image in devices of the above-described type, luminious fluxes belonging to the visible range of light are used. Measurements with wave lengths of the invisible ranges or ranges closely adjacent to the visible of light would require expensive and time-consuming modifications of the above-described devices.